Members of the Hedgehog (Hh) family of signaling molecules mediate many important short and long range patterning processes during invertebrate and vertebrate development. Pattern formation is the activity by which embryonic cells form ordered spatial arrangements of differentiated issues. Hedgehog proteins were first discovered in Drosophila. Although some crucial differences exist, the signalling mechanisms are generally well conserved between Drosophila and higher organisms. In the fly, a single Hh gene regulates segmental and imaginal disc patterning. In contrast, in vertebrates, an Hh gene family is involved in the control of left-right asymmetry, polarity in the CNS, somites and limb, organogenesis, chondrogenesis and spermatogenesis. Three Hh homologues have been identified in humans: Sonic hedgehog (SHH), Indian hedgehog (IHH) and Desert hedgehog (DHH). The Hh signaling cascade is initiated by Hh binding to the Patched proteins (PTCH1 in humans) on the target cell. In the absence of the Hh ligand, PTCH1 represses the activity of Smoothened (SMO in humans, Smo in mouse and smo in Drosophila), a G-protein-coupled receptor (GPCR)-like protein. Mammalian Hh signaling requires the presence of non-motile cilia to which SMO and other downstream pathway components localize to achieve activation of GLI transcription factors, the cubis interruptus (Ci) orthologues. The activator and repressor forms of Ci in mammals are represented by three separate zinc-finger proteins, with GLI1 and GLI2 functioning mostly as activators and GLI3 as a repressor. For review, see Rubin L. L. et al. (2006) Nature Reviews, vol 5, 1026-1033. The mechanism by which this signaling cascade regulates proliferation involves the activation of cyclins and cyclin-dependent kinases. The control of differentiation might be occurring via the production of other secreted proteins, including neurotrophic and angiogenic factors.
Medicinal chemistry efforts to identify inhibitors of Hh pathway began when Richard Keeler and co-workers isolated teratogens from Veratrum califormicum in 1964. Subsequent research established that the previously known alkaloid jervine and the newly discovered alkaloid cyclopamine were able to induce cyclopia. Almost four decades later, the heptahelical bundle of Smo was identified as the site of binding of cyclopamine using its photoaffinity and fluorescent derivatives. Chen, J. et al. (2002) Genes & Develop. 16: 2743-2748; Chen, J. et al. (2002) Proc. Natl. Acad. Sci. USA 99: 14071-14076; Frank-Kamenetsky, M. et al. (2002) J. Biol. 1, article 10. Several assays are used to screen for antagonists to Smo in vitro. One of the assays for high throughput screening examines the overall activity of the Hh pathway in a cellular context by determining the degree of activity of the downstream effector protein GLI. Chen et al., supra. Cell lines of this type often incorporate a GLI dependent luciferase reporter for the assay readout. The luciferase signal may be boosted by other engineer modifications, such as the addition of biologically active Shh, (e.g., Shh with an octyl moiety attached to its N terminus), or the utilization of cell lines that lack PTCH1 function. Alternatively, direct binding to Smo can be measured through the displacement of a fluorescent cyclopamine derivative. In addition, tumor xenograft models based on SCLC, biliary, prostate, pancreatic and medulloblastoma lines can also be used.
In the recent years it was established that aberrant activation of the Hh signaling pathway can lead to cancer. Gorlin syndrome (GS), or nevoid basal cell carcinoma syndrome, is an autosomal dominant genetic disease that is characterized by development abnormalities and tumor predisposition. Virtually all individuals with Gorlin syndrome develop basal cell carcinomas (BCC), usually at multiple sites, and are predisposed to other kinds of cancer as well, especially medulloblastoma, a tumor of cerebellar granule neuron progenitor cells, rhabdomyosarcoma, a muscle tumor, as well as ovarian fibromas and sarcomas. Borzillo, G. et al. (2005) Curr. Topics in Med. Chem. 5: 147-157.
BCC is the most common human cancer, accounting for about 70% of human skin cancers, and representing at least one third of all cancer diagnosed in the US each year. More than 99% of BCC cases arise sporadically in the population, with only 0.5% of cases arising in individuals with GS. BCC rarely metastasizes, but can be locally aggressive and recurrent. Inactivating mutations in PTCH1 occur most commonly in these tumors. A subset of BCC is driven via mutations in SMO, and these mutations activate the pathway by generating proteins with decreased sensitivity to PTCH1 suppression.
Medulloblastoma (MB) is a brain tumor that forms in the cerebellum of children and young adults, and may be the end result of defect in cerebellar organogenesis. MB, in addition to BCC, has a well recognized involvement of the Hh pathway. The outcome of this cancer is almost invariably poor. Surgery with subsequent radiation or chemotherapy increases survival to greater than 50%, but there is severe treatment-associated morbidity, including mental retardation. Hh-pathway antagonists have been tested in cell culture and mouse models of medulloblastoma. A new class of SMO-binding Hh antagonists has been demonstrated to be very potent. Berman C. M. et al. (2002) Science 297: 1559-1561.
Hh pathway has been implicated in many other types of cancer, including pancreatic cancer, other tumors of the gastrointestinal (GI) tract and prostate cancer. Abnormal expression of SHH, PTCH1 and SMO has been shown early in the formation of human pancreatic tumors. Thayer, S. P. et al. (2003) Nature 425: 313-317. Several pancreatic cancer cell lines were found to be PTCH1 and SMO-positive and growth inhibited in vitro by cyclopamine, suggesting an active autocrine loop through which tumor cells both make and respond to Hh ligand. Furthermore, systemic treatment with cyclopamine slowed the growth of tumors formed when these cell lines are implanted into immunocompromised mice. Similar observations were made for pancreatic and other GI tumors. Berman, D. M. et al. (2003) Nature 425: 846-851. Similar data was provided for prostate cancer as well, including SHH overexpression in tumor biopsies, especially in higher Gleason grade tumors, and in vitro and in vivo inhibitory effects of cyclopamine on growth of prostate cancer cell lines. The Hh pathway was further implicated in prostate tumor metastasis, as the capacity of AT6.3 cells to metastasize to the lung was completely abrogated by cyclopamine, and AT2.1, a rarely metastasizing clone, could be induced to metastasize by overexpression of GLI1, in a cyclopamine-insensitive manner.
It has been demonstrated recently that Hh may be involved in the development of a significant subset of small cell lung carcinoma (SCLC). Watkins, D. N. et al. (2003) Nature 422: 313-317. In this study, Shh pathway components were found to be reactivated in a mouse model of acute airway damage caused by naphthalene. About 50-70% of SCLC lines and primary tumors expressed transcripts (SHH, PTCH1, GLI1) indicative of activated Shh signaling. Cyclopamine blocked the growth of only those cells with persistent Hh signaling, and this effect was abrogated by overexpression of GLI1. None of the effects of cyclopamine could be reproduced with tomatidine, a compound that is structurally similar but inactive against SMO.
These results demonstrate that Hh pathway is an important pharmacological target for a variety of cancers. Compounds and compositions of the current invention present an important treatment option for all tumors driven by inappropriate Hh signaling.